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// Copyright 2005 Google Inc. All Rights Reserved.
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#include "base/integral_types.h"
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#include "base/logging.h"
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#include "s2latlngrect.h"
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// Multiply a positive number by this constant to ensure that the result
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// of a floating point operation is at least as large as the true
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// infinite-precision result.
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double const kRoundUp = 1.0 + 1.0 / (uint64(1) << 52);
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// Return the cap height corresponding to the given non-negative cap angle in
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// radians. Cap angles of Pi radians or larger yield a full cap.
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double GetHeightForAngle(double radians) {
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DCHECK_GE(radians, 0);
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// Caps of Pi radians or more are full.
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if (radians >= M_PI) return 2;
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// The height of the cap can be computed as 1 - cos(radians), but this isn't
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// very accurate for angles close to zero (where cos(radians) is almost 1).
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// Computing it as 2 * (sin(radians / 2) ** 2) gives much better precision.
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double d = sin(0.5 * radians);
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S2Cap S2Cap::FromAxisAngle(S2Point const& axis, S1Angle const& angle) {
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DCHECK(S2::IsUnitLength(axis));
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DCHECK_GE(angle.radians(), 0);
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return S2Cap(axis, GetHeightForAngle(angle.radians()));
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S1Angle S2Cap::angle() const {
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// This could also be computed as acos(1 - height_), but the following
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// formula is much more accurate when the cap height is small. It
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// follows from the relationship h = 1 - cos(theta) = 2 sin^2(theta/2).
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if (is_empty()) return S1Angle::Radians(-1);
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return S1Angle::Radians(2 * asin(sqrt(0.5 * height_)));
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S2Cap S2Cap::Complement() const {
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// The complement of a full cap is an empty cap, not a singleton.
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// Also make sure that the complement of an empty cap has height 2.
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double height = is_full() ? -1 : 2 - max(height_, 0.0);
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return S2Cap::FromAxisHeight(-axis_, height);
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bool S2Cap::Contains(S2Cap const& other) const {
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if (is_full() || other.is_empty()) return true;
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return angle().radians() >= axis_.Angle(other.axis_) +
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other.angle().radians();
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bool S2Cap::Intersects(S2Cap const& other) const {
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if (is_empty() || other.is_empty()) return false;
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return (angle().radians() + other.angle().radians() >=
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axis_.Angle(other.axis_));
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bool S2Cap::InteriorIntersects(S2Cap const& other) const {
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// Make sure this cap has an interior and the other cap is non-empty.
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if (height_ <= 0 || other.is_empty()) return false;
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return (angle().radians() + other.angle().radians() >
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axis_.Angle(other.axis_));
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void S2Cap::AddPoint(S2Point const& p) {
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// Compute the squared chord length, then convert it into a height.
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DCHECK(S2::IsUnitLength(p));
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// To make sure that the resulting cap actually includes this point,
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// we need to round up the distance calculation. That is, after
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// calling cap.AddPoint(p), cap.Contains(p) should be true.
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double dist2 = (axis_ - p).Norm2();
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height_ = max(height_, kRoundUp * 0.5 * dist2);
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void S2Cap::AddCap(S2Cap const& other) {
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// See comments for AddPoint(). This could be optimized by doing the
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// calculation in terms of cap heights rather than cap opening angles.
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double radians = axis_.Angle(other.axis_) + other.angle().radians();
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height_ = max(height_, kRoundUp * GetHeightForAngle(radians));
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S2Cap S2Cap::Expanded(S1Angle const& distance) const {
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DCHECK_GE(distance.radians(), 0);
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if (is_empty()) return Empty();
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return FromAxisAngle(axis_, angle() + distance);
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S2Cap* S2Cap::Clone() const {
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return new S2Cap(*this);
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S2Cap S2Cap::GetCapBound() const {
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S2LatLngRect S2Cap::GetRectBound() const {
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if (is_empty()) return S2LatLngRect::Empty();
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// Convert the axis to a (lat,lng) pair, and compute the cap angle.
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S2LatLng axis_ll(axis_);
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double cap_angle = angle().radians();
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bool all_longitudes = false;
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double lat[2], lng[2];
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// Check whether cap includes the south pole.
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lat[0] = axis_ll.lat().radians() - cap_angle;
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if (lat[0] <= -M_PI_2) {
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all_longitudes = true;
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// Check whether cap includes the north pole.
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lat[1] = axis_ll.lat().radians() + cap_angle;
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if (lat[1] >= M_PI_2) {
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all_longitudes = true;
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if (!all_longitudes) {
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// Compute the range of longitudes covered by the cap. We use the law
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// of sines for spherical triangles. Consider the triangle ABC where
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// A is the north pole, B is the center of the cap, and C is the point
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// of tangency between the cap boundary and a line of longitude. Then
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// C is a right angle, and letting a,b,c denote the sides opposite A,B,C,
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// we have sin(a)/sin(A) = sin(c)/sin(C), or sin(A) = sin(a)/sin(c).
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// Here "a" is the cap angle, and "c" is the colatitude (90 degrees
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// minus the latitude). This formula also works for negative latitudes.
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// The formula for sin(a) follows from the relationship h = 1 - cos(a).
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double sin_a = sqrt(height_ * (2 - height_));
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double sin_c = cos(axis_ll.lat().radians());
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if (sin_a <= sin_c) {
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double angle_A = asin(sin_a / sin_c);
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lng[0] = drem(axis_ll.lng().radians() - angle_A, 2 * M_PI);
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lng[1] = drem(axis_ll.lng().radians() + angle_A, 2 * M_PI);
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return S2LatLngRect(R1Interval(lat[0], lat[1]),
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S1Interval(lng[0], lng[1]));
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bool S2Cap::Intersects(S2Cell const& cell, S2Point const* vertices) const {
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// Return true if this cap intersects any point of 'cell' excluding its
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// vertices (which are assumed to already have been checked).
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// If the cap is a hemisphere or larger, the cell and the complement of the
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// cap are both convex. Therefore since no vertex of the cell is contained,
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// no other interior point of the cell is contained either.
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if (height_ >= 1) return false;
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// We need to check for empty caps due to the axis check just below.
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if (is_empty()) return false;
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// Optimization: return true if the cell contains the cap axis. (This
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// allows half of the edge checks below to be skipped.)
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if (cell.Contains(axis_)) return true;
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// At this point we know that the cell does not contain the cap axis,
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// and the cap does not contain any cell vertex. The only way that they
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// can intersect is if the cap intersects the interior of some edge.
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double sin2_angle = height_ * (2 - height_); // sin^2(cap_angle)
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for (int k = 0; k < 4; ++k) {
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S2Point edge = cell.GetEdgeRaw(k);
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double dot = axis_.DotProd(edge);
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// The axis is in the interior half-space defined by the edge. We don't
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// need to consider these edges, since if the cap intersects this edge
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// then it also intersects the edge on the opposite side of the cell
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// (because we know the axis is not contained with the cell).
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// The Norm2() factor is necessary because "edge" is not normalized.
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if (dot * dot > sin2_angle * edge.Norm2()) {
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return false; // Entire cap is on the exterior side of this edge.
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// Otherwise, the great circle containing this edge intersects
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// the interior of the cap. We just need to check whether the point
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// of closest approach occurs between the two edge endpoints.
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S2Point dir = edge.CrossProd(axis_);
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if (dir.DotProd(vertices[k]) < 0 && dir.DotProd(vertices[(k+1)&3]) > 0)
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bool S2Cap::Contains(S2Cell const& cell) const {
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// If the cap does not contain all cell vertices, return false.
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// We check the vertices before taking the Complement() because we can't
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// accurately represent the complement of a very small cap (a height
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// of 2-epsilon is rounded off to 2).
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for (int k = 0; k < 4; ++k) {
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vertices[k] = cell.GetVertex(k);
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if (!Contains(vertices[k])) return false;
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// Otherwise, return true if the complement of the cap does not intersect
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// the cell. (This test is slightly conservative, because technically we
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// want Complement().InteriorIntersects() here.)
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return !Complement().Intersects(cell, vertices);
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bool S2Cap::MayIntersect(S2Cell const& cell) const {
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// If the cap contains any cell vertex, return true.
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for (int k = 0; k < 4; ++k) {
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vertices[k] = cell.GetVertex(k);
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if (Contains(vertices[k])) return true;
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return Intersects(cell, vertices);
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bool S2Cap::Contains(S2Point const& p) const {
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DCHECK(S2::IsUnitLength(p));
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return (axis_ - p).Norm2() <= 2 * height_;
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bool S2Cap::InteriorContains(S2Point const& p) const {
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DCHECK(S2::IsUnitLength(p));
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return is_full() || (axis_ - p).Norm2() < 2 * height_;
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bool S2Cap::operator==(S2Cap const& other) const {
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return (axis_ == other.axis_ && height_ == other.height_) ||
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(is_empty() && other.is_empty()) ||
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(is_full() && other.is_full());
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bool S2Cap::ApproxEquals(S2Cap const& other, double max_error) {
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return (S2::ApproxEquals(axis_, other.axis_, max_error) &&
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fabs(height_ - other.height_) <= max_error) ||
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(is_empty() && other.height_ <= max_error) ||
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(other.is_empty() && height_ <= max_error) ||
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(is_full() && other.height_ >= 2 - max_error) ||
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(other.is_full() && height_ >= 2 - max_error);
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ostream& operator<<(ostream& os, S2Cap const& cap) {
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return os << "[Axis=" << cap.axis() << ", Angle=" << cap.angle() << "]";